Most solar cells absorb parts of sunlight to have the photovoltaic (PV) effect. The raw materials for solar cells contain mostly silicon, because silicon is the second most easily accessible chemical element on earth, and silicon has advantages of low cost, no toxicity and high stability. Further, silicon is widely used in the semiconductor industry.
Silicon materials for solar cells include mono-crystalline silicon, poly-crystalline silicon and amorphous silicon. In General, poly-crystalline silicon is chosen due to its lower cost, comparing with the mono-crystalline silicon that is made from conventional growth techniques (e.g. the Kyropolus and Czochralski (CZ) growth method and the floating zone method (FZ) method).
In general, a solar cell made of poly-crystalline silicon is made by using a known technique. In short, the process of the technique is first to put the high purity silicon melt in a mold (i.e. a quartz crucible), then cool the mold to solidify the silicon melt into a silicon ingot. The silicon ingot then is cut into slices of a wafer size for solar cell applications.
During the poly-crystalline silicon ingots manufacturing process, defects exist among silicon grains, which reduce the solar cell conversion efficiency. The defects include, for example, dislocation, grain boundary, etc., have high probability of forming a recombination center to reduce the lifetime of minority carriers (e.g. electronics). In other words, the defect density in multi-crystalline silicon greatly impacts component structures, characteristics and carrier transportation speeds. The prior art mostly focuses on making poly-crystalline silicon ingot with a low defect density or with harmless defects (e.g. twin boundary etc.).
However, no known prior-art technique has been introduced for reducing the crucible contamination to the edges of the crystalline silicon ingot which contacts with the mold and have to remove as waste due to the contamination.
In addition, most known manufacturing methods for silicon crystalline ingots use the mono-crystalline silicon seeds. The mono-crystalline silicon seeds account for a high percentage to overall manufacturing cost. Also, in solar cell industry, no prior-art technique has been found using low defect density poly-crystalline silicon as seeds to replace the mono-crystalline silicon, which lowers the cost for seeds used in silicon crystalline ingots fabrication.
Further, known methods for manufacturing a silicon ingot with a low defect density or harmless defects have high manufacturing cost.